As a format for compressing an audio signal, a lossy compression format, such as, MP3 (MPEG Audio Layer-3), WMA (Windows Media Audio™), and AAC (Advanced Audio Coding), is known. Regarding the lossy compression format, a high compression rate is attained by significantly cutting a high frequency component close to an upper limit of an audible band or exceeding the upper limit of the audible band. At the beginning of the period where technology of this type was developed, it was believed that, even when a high frequency component is cut significantly, sound quality in terms of auditory feeling is not deteriorated. However, in recent years, the thought that cutting significantly a high frequency component causes minute changes in sound quality and thereby sound quality in terms of auditory feeling is deteriorated in comparison with original sound has become the mainstream. In view of the circumstances, a high band interpolating, apparatus which enhances sound quality by interpolating a high band for an audio signal which has been subjected to a lossy compression. A specific configuration of a high band interpolating apparatus of this type is described, for example, in Japanese Patent Provisional Publication No. 2007-25480A (hereafter, referred to as patent document 1) and Domestic re-publication of PCI publication No. 2007-29796A1 (hereafter, referred to as patent document 2).
The high band interpolating apparatus described in the patent document 1 calculates a real part and an imaginary part of a signal obtained by analyzing an audio signal (original signal), forms an envelope component of the original signal based on the calculated real part and the imaginary part, and extracts a higher harmonic component of the formed envelope component. The high band interpolating apparatus described in the patent document 1 executes interpolation for a high band of the original signal by synthesizing the extracted higher harmonic component and the original signal.
The high band interpolating apparatus described in the patent document 2 inverts a spectrum of an audio signal, upsamples the signal of which spectrum is inverted, and extracts an expanded band component of which the lower frequency edge is approximately equal to a high band of a baseband signal based on the upsampled signal. The high band interpolating apparatus described in the patent document 2 executes interpolation for a high band of the baseband signal by synthesizing the extracted expanded band component and the baseband signal.